D.c. protection and control panel



Dec. 8, 1970 D. L. EISENSTADT I D.C. PROTECTION AND CONTROL PANEL 5Sheets-Sheet 2 Filed Dec. 4, 1967 N\ r wl J L r um n@ wd ..v\ @n NMM mwNo n ma III. \\.1 Nh; LT w .mv w\ w o \N F NW Nw L Dec 8, 1970 l Y D. L.EISENSTADT 3,546,532

D.C. PROTECTION AND CONTROL PANEL Filed Dec. 4. 1967 5 Sheets-Sheet 3x74/ @JC :l Z y il y l .l

. 7.3 i i l I l l l l i I l I I I l l l l 30 *'76 .-76 ,-/7 H76INVENTOR. 04h/fo MEA/57407 lay/50M wf) @Jaw y/ Dec. 8, 1970 Filed Dec.4. 1967 D. L. EISENSTADT 3,546,532

D.C. PROTECTION AND CONTROL PANEL 5 Sheets-Sheet 4 INVENTOR. OAV/0ua/57,407

Dec, v8, 1970 Filed Dec. 4. 1967 D. L. EISENSTADT D.C. PROTECTION ANDCONTROL PANEL 5 Sheets-Sheet 5 INVENTOR. OAV/O EASEA/STOT United StatesPatent O 3,546,532 DC. PROTECTION AND CONTROL PANEL David L. Eisenstadt,Bedford, Ohio, assignor to Lear Siegler, lne., Santa Monica, Calif., acorporation of Delaware Filed Dec. 4, 1967, Ser. No. 687,667 Int. Cl.H0211 3/00, 7/00 U.S. Cl. 317-13 5 Claims ABSTRACT OF THE DISCLOSUREBACKGROUND OF THE INVENTION This invention relates to control andprotection of direct current generators including D.C. generators of thetype which may be operated in parallel in an aircraft electrical system.Operation of such generators and generator systems includes control oflocal generator output voltage, equalization of the load among theseveral generators on the line and protection against current flow fromline to generator. Such a control and protection system accomplishesthese and other functions by controlling generator field excitation,connection of the local generators to the main load bus, and connectionof the local generators to the equalizing bus.

In the past, control circuits for direct current generators haveemployed control and sensing elements which are rather large and heavyfor airborne use. Voltage regulation of a D.C. generator, for example,is accomplished conventionally by a carbon pile regulator connected toregulate the generator field excitation. Such a device normally consistsof a pile of carbon discs, the total resistance of which variesinversely with the degree of cornpression applied. A proportioning typeelectromagnetic actuator responds to the output voltage of the generatorand controls the amount of compression, thereby varying proportionatelythe resistance of the carbon pile. As a result, more or less currentiiows to the exciter iield of the generator and increases or decreasesthe generator output voltage. This type of regulator is heavy andrelatively slow acting. Furthermore, it is inefficient because it isrequired to dissipate considerable wattage in its resistance.

Similarly, conventional controls for providing load equalization,overvoltage protection, differential protection and main contactorcontrol are normally accomplished by large, expensive, slow-acting,specially-constructed relays.

For example, connection of the local generator of a parallel generatorsystem to the main load bus is generally accomplished by a main linecontactor. Before a local generator can be put on the line, however,there should be no undesirable voltage difference between the main loadbus and the local generator. Once the local generator is connected tothe main load bus, it is necessary to sense and indicate the flow of anyreverse current to the local generator indicating its having become aload on the parallel system instead of a contributor to it. In the past,separate circuits or a special complex relay has been required to sensevoltage difference between the rnain bus and the local generator and tosense reverse Patented Dec. 8, 1970 current flowing to the localgenerator after connection to the mian bus. Such a relay, however, isexpensive, heavy, and extremely delicate.

Also, the means for disconnecting the local generator from the parallelsystem in case of overvoltage or a ground fault of some type isconventionally a speciallybuilt relay adapted to sense the overvoltageor the difference in current in different parts of the system caused bythe ground fault. Such a relay normally is also heavy, expensive, anddelicate. In all the specially-built relays mentioned above and used fordisconnecting the local generator from all or part of the system, abuilt-in weighting factor is required to determine that the sensedconditions actually exist and continue for a predetermined length oftime and that a mere transient signal has not been sensed. These relaysare required, in effect, to integrate a sensed condition over a smallperiod of time to determine that it actually exists. This requirementadds to the size and expense of such special relays.

To a limited extent, the disadvantages of size and weight associatedwith the electromechanical control devices referred to above have beenreduced through the use of elemental solid state devices such as powertransistors, silicon controlled rectiliers, and Zener diodes. Suchsavings are achieved, however, at a sacrifice in mechanical simplicityexchanged for elaborate circuits designed to utilize the solid statedevices as control and sensing elements. Furthermore, when such solidstate defvices are used and operated in their active region asproportional controllers, they are required to dissipate wattage asheat. Excessive heat can cause thermal runaway and require largeheat-absorbing surfaces or cooling devices for acceptable operation.Solid state devices thus oifer only a partial solution to some of theproblems of the electromechanical devices.

Solid state devices such as those mentioned above sometimes can beoperated as switches rather than proportioning devices which reduces theamount of power they are required to dissipate and thereby increasestheir efiiciency. One type of regulator that uses this principle dependsupon the generators ability to follow load conditions and swtiches theeld on and olf as required to maintain desired terminal voltage. Undersuch conditions, for example, a D.C. generator is controlled by havingits eld fully excited or unexcited as required by the load and thegenerators ability to recover and hold terminal voltage. A disadvantageof this type of regulator is low frequency of correction causing highline ripple. This ripple tends to produce a high level of radio noiseand poor quality power.

This type of control has been found to be particularly unsatisfactory incontrolling a parallel system of D.C. generators supplying current to acommon load. Normally, the on or off state of the field excitation meansis determined by the output level of the generator and the amount ofload it is carrying. In such a case, the local generator calls forchanges in excitation at a rate dependent upon the generators inherentspeed of response to the excitation. An operational frequency of theon-oif excitation means is thus established that is related to thenatural resonant response frequency of the generator. This phenomenoncauses a problem when two or more generators so controlled areparalleled to the same load because normal differences in theirrespective speeds of response to excitation cause a load unbalancebetween the generators and set up oscillation of load between them.

SUMMARY OF THE INVENTION A preferred embodiment of this inventioncomprises cooperating and interrelated circuit means to control theoperation of a local D.C. generator acting alone and in a system ofparalleled generators in accordance with predetermined operatingconditions and in an appropriate and predetermined manner under faultconditions which may exist or occur in the system.

The circuit means includes a field excitation control circuit of theon-off type in which a solid state switch is controlled to feed currentpulses at a relatively high frequency and of varying widths to the fieldwinding of the local generator in a pulse Width modulation arrangement.The pulsing switch is controlled by an integrated circuit linearoperational amplifier operated as a bistable device. The bistable deviceis provided with a differential input and is responsive to sensed andalgebraically combined indications of generator output voltage, arelatively high frequency pulsing signal, and a load equalization signaland the relationship of this combined signal to a reference level. Theload equalization signal is provided by another integrated circuitlinear operational amplifier operated as a modified differentialamplifier and responsive to the sensed voltage difference resulting fromany unequal sharing of the load by the local generator and otherparalleled generators of the system. A unijunction oscillator providesthe pulsing signal at a 1000 cycle per second rate, for example. Thepulsing signal is a periodic sawtooth signal which serves to pulse theexcitation switch once each cycle with a pulse of current whose durationdepends upon the generator output voltage signal and the loadequalization signal. The excitation current pulses are supplied to thefield winding of the local generator at a rate of 1000 cycles persecond. Although the excitation is of the on-off type, the rate ofexcitation is so much faster than the normal response time ofconventional on-off type regulators that the generator responds asthough it Were receiving constant proportional excitation and enjoyingall the benefits associated therewith. Thus, the field excitationcontrol circuit of this invention combines the advantages of aproportional controller and of an on-o-ff type controller to solve theproblems and remedy the disadvantages inherent in conventionalapplications of each of those control methods.

The automatic line contactor control circuit comprises the combinationof a first differential input integrated circuit linear operationalamplifier operated as a bistable device for providing on-off control ofthe main contactor connecting the generator to the load bus and a seconddifferential input integrated circuit linear operational amplifierconnected as an integrator to perform the dual functions of sensing thevoltage between main load bus and local generator before the mainconductor is closed and sensing reverse current flow into the localgenerator after the main contactor is closed. When the main contactor isopen, the differential input of the first operational amplifier sensesthe voltage difference between the main load bus and the localgenerator. When the voltage of the local generator equals or exceedsvoltage of the main bus, the second and integrating operationalamplifier begins to integrate; and, if the condition is maintained, theintegrated output causes the energization of the main contactor coil andthereby connects the local generator to the load bus. When the maincontactor is closed, the first differential input operational amplifiersenses any voltage difference between its terminals caused by reversecurrent flow to the local generator. When such a condition is detected,the second and integrating operational amplifier begins to integrate thesignal; and, if the condition maintains itself long enough, theintegrated signal will cause the main contactor to be de-energized,disconnecting the local generator from the main load bus and from theequalizer bus. The automatic line contactor control circuit of thisinvention combines a differential input integrator and on-off typecontrol elements to control the main contactor in an advantageous,efficient, and reliable manner and without special relays. In addition,the circuit performs both the function of differential voltage sensingand the function of reverse current sensing.

The field relay trip control circuit disclosed senses fault conditionsin the system of which the local generator is an element, and controlsconnection of the generator to the system in accordance with apredetermined desired course of action. A preferred embodiment of thefield relay trip control circuit includes solid state switches fordisconnecting the generator from the line, means for detecting localground faults, a first differential input integrating operationalamplifier for sensing start ground faults, and a second differentialinput integrating operational amplifier responsive to sensed conditionsof overvoltage, overexcitation and a voltage regulator failure, and astart ground fault for disconnecting the local generator from theparallel system when an undesirable condition continues for apredetermined length of time. Local ground faults developing after thelocal generator is connected to the main load bus are detected 'by acurrent transformer sensing load current at a point as near as possibleto the local generator and another sensing load current at a point asnear as possible to the main load bus. Any difference in load currentdetected by the two current transformers actuates means to disconnectthe local generator from the system and the load fault condition fromthe system.

In addition to the individual functions of each circuit described above,the separately described circuits interact and cooperate to complementand supplement the principal functions of each circuit. Means areincluded, for example, to implement predetermined fail-safe alternativesin the event the control circuitry or some portion of it ceases tofunction in the normal manner.

Throughout the embodiment of this invention described below, operationalamplifiers, and particularly differential input integrating amplifiers,are used as sensing and control elements in conjunction with discretesolid state switching elements to gain the advantages and substantiallyavoid the disadvantages of the electromechanical proportional typecontrol devices and the solid state on-off devices. By the use of thedifferential input integrating ampliers, proportional weighting isachieved through signal integration without the use of heavy andexpensive special relays. These and other features and advantages ofthis invention are disclosed and described below.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates in combinedschematic and diagrammatic form a parallel D.C. generator system and thecontrol and protection system of this invention and the overallrelationship between them;

FIG. 2 is a schematic representation of the field excitation controlsection and paralleling section of the system;

FIGS. 3A through 3F are diagrams of input and output voltage wave formsas they occur under different operating conditions in the fieldexcitation control circuit of FIG. 2;

FIG. 4 is a schematic diagram of the automatic line contactor controlsection of the system; and

FIG. 5 is a schematic diagram of the field relay trip control section ofthe system.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred form and embodimentof this invention and its relationship to a typical D.C. generator and aparallel D.C. generator system is depicted in FIG. l. Broken outlineboxes 5, 6, and 7 represent a plurality of identical local D.C.generators; and, broken outline boxes 8, 9, and 10, respectively,represent identical control and protection panels embodying thisinvention and associated with the generators. Local generators 5, 6, and7 are each connected in parallel with each other to main bus L and,through their associated control and protection panels, to equalizer busE. Since the generators and their associated control and protectionpanels are the same, only local generator 5 and its associated panel 8are shown in schematic and diagrammatic detail. It will be understoodthat any number of such generators and control and protection panels maybe connected together in the manner shown to main bus L and equalizerbus E.

Local generator includes an armature 11, an interpole winding 12, and ashunt field winding 13. Local generator 5 is provided with generatorterminals 14 and 15, shunt field terminal 16 and neutral terminal 17,the latter being grounded.

Armature 11 is connected by means of line 18 to one terminal of maincontactor 19. The other terminal of contactor 19 is connected to loadbus L. Main contactor 19 is actuated by coil 21. In a typicalinstallation, generator 5 may be located a significant distance frommain contactor 19.

A current transformer indicated generally at 22 is inductivelyassociated with line 18 as near as practicable to main contactor 19.Another current transformer indicated generally at 23 is connected asnear as possible to the neutral terminal side of local generator 5. Thesecondary windings of the two current transformers 22 and 23 areconnected in series opposing and to ground fault sensing circuitry ofthe field trip control circuit as described more fully below. Otherlocal D.C. generators 6 and 7 are connected in the same fashion to loadbus L and thereby in parallel with local generator 5.

In addition to main contactor 19, there are three switching relaysassociated with generator 5 and which comprise conventional elements ofthe control and protection system included generally within brokenoutline box 8. They are the field relay 26, equalizer relay 27 and startfault relay 28. Field relay 26 is a latching relay having two separatecoils, a trip coil and a reset coil and 31, respectively. When fieldrelay 26 is operated, it connects shunt field 13 to its source ofexcitation and also provides a means for energizing the actuating coilsof rnain contactor 19 and equalizer relay 27. When field relay 26 istripped,

local generator 5 is de-energized and disconnected from the parallelsystem.

Equalizer relay 27 has an actuating coil 32 connected to be energizedwith main contactor actuating coil 21. Equalizer relay 27 connectsterminal 15 of local generator 5 through equalizer bus E to thecorresponding armature terminals of other generators with whichgenerator 5 is connected in parallel. The equalizer bus Einterconnection provides means to determine the relative amount ofcurrent supplied by the local generator 5 and the other generators 6 and7 connected to the equalizer bus. Generator terminal 15 of localgenerator 5 is a convenient point for sensing the amount of loadsupplied by the generator as will be more fully disclosed hereinafter.

Start fault sensing relay 28 provides a path for detecting a localground fault before the local generator 5 is connected to main bus L andoperates as described more fully below.

The system for controlling and protecting a typical local generator andassociated switching relays such as generator 5 and relays 26, 27, and28 described above must perform a number of control functions. Thecontrol system must determine, for example, the appropriate time toconnect the local generator to the main bus. To to so, the controls mustdetermine that there is no appreciable voltage difference between themain Ibus and the local generator. Once the generator is connected, itmust be determined that the generator is supplying current to the loadand not taking current from the load bus. It must also be determinedthat the local generator is supplying its share of the current suppliedby all the generators on the line. The output voltage level of the localgenerator must also be maintained at a predetermined desired level. Thecontrol panel must sense fault conditions such as a local ground fault;i.e., a ground fault on the generator side of its associated maincontactor connecting it to the main bus. Such a fault causes thegenerator to supply a large amount of current to the fault instead of tothe main bus. Likewise, a remote ground fault on the main bus causingthe entire generator system to supply current to that ground faultinstead of to the proper load must be sensed and the appropriate actiontaken. Numerous other conditions and faults may arise which requireautomatic sensing and control action from the control circuits.

The functions described above and other functions are performed in areliable and advantageous manner by the preferred form of control andprotection panel 8 described below and comprising three principalcontrol sections which interact together and with local generator 5 andits associated switching relays, These sections are the automaticcontactor control 34, the field excitation control 35, and the fieldrelay control 36. In FIG. 1, these control sections are shown in blockform within control and protection panel 8. They are shown in detail inother figures of the drawings and each is fully described below inconnection with them.

All power to the principal sections of the D.C. generator control panelis supplied by its associated local D.C. generator.

FIELD EXCITATION CONTROL CIRCUIT FIG. 2 is a schematic diagram of apreferred form of field excitation control section 35 in detail and inconnection with local generator 5 shown in FIG. 1 and described above.Power for this section, as well as all sections of the control andprotection panel, is supplied by the associated local generator. In FIG.'2, a positive bus 38 is connected to generator terminal 14 and agrounded bus 39, to neutral terminal 17. Control section 35 comprises anintegrated circuit linear operational amplifier 40 connected as adifferential switch amplifier and having a summing input 41, a referenceinput 42, and an output 43. Broadly, operational amplifier 40 controlsthe conductive state of a transistor switch controlling the connectionof shunt field 13 of generator 4 across generator terminals 14 and 15and thus the field excitation of the generator. Operational amplifier 40responds to voltages applied to its inverting and non-inverting inputsas described more fully below.

Operating power is supplied to operational amplifier 40 in aconventional manner by connection to positive bus 38 and grounded bus39. The gain and biasing of operational amplifier 40 are such that itoperates as a bistable device providing a substantially square-waveoutput. Summing input 41 is an inverting input with respect to output43. Reference input 42 is a non-inverting input which is connected to asource of reference voltage ER as shown. Operational amplifier 40 istriggered to one of the stable states providing a more positive voltageat output 43 whenever the voltage at summing input 41 is less than thevoltage ER at reference input 42. When the voltage at summing input 41is greater than the reference voltage ER at input 42, amplifier 40assumes its less positive output state.

Output 43 of operational amplifier 40 is connected through the seriescombination of Zener diode 44 and resistor 45 to the base 46 of PNPtransistor v48. Base 46 is also connected through resistor 47 topositive bus 38. Emitter 49 and collector 5t)v of transistor 48 areconnected to positive bus 38 and grounded bus 39, respectively, thelatter through resistor 51.

Shunt field winding 13 of local generator 5 is excited from generatorterminal 14 through the collector-emitter circuit of PNP transistor 53of the Darlington-connected pair 52 and 53. The collector 55 oftransistor 53 is connected to contact 56 of field relay 26 and therebyto shunt field terminal 16.

The network described above and connected to output 43 of operationalamplifier including transistor 48 and Darlington-connected pair oftransistors 52 and 53 function to connect and disconnect shunt field 13and generator terminal 14 of local generator S in accordance with thestable state assumed by operational amplifier 40. When the output ofoperational amplifier 40 is more positive, conduction by Zener diode 44is blocked and transistor 48 is held non-conducting by the resultingpositive voltage level at its base 46. When transistor 48 is blocked,Darlington pair S2 and 53 conducts and shunt field 13 is excited fromgenerator terminal 14 of its local generator 5. When the output ofoperational amplifier 40 is less positive, Zener diode 44 conducts,providing a lower voltage level at base 46 and permitting transistor 48to conduct. When transistor 48 conducts, Darlington pair 52 and 53 isheld non-conducting and shunt field 13 is disconnected from generatorterminal 14. A return diode 57 is connected in parallel with shunt field13 as shown to provide a path for an induced current to circulate whenexcitation of the shunt field is removed.

The voltage at summing input 41 is the sum of three voltages introducedthrough resistors 58, 59, and 60 as shown in FIG. 2. The sum of thevoltages at these three inputs determines whether or not the voltagelevel at summing input 41 iS lower or higher than the level ER atreference input 42. The voltages applied to summing input 41 throughresistors 58 and 59 may be considered primarily as voltage regulatorsignals while the input through resistor 60 is a load regulation signal.

The voltage coming to summing input 41 through resistor 59 is obtainedby sampling the local generator voltage directly through a voltagedivider arrangement of resistors 61, 62, and 63 connected acrosspositive bus 38 and grounded bus 39.

The voltage supplied to summing input 41 through resistor 58 has asaw-tooth waveform delivered from emitter 64 of unijunction transistor66. Unijunction transistor 66, resistor 67 and capacitor 69 comprise anoscillator providing a saw-tooth waveform output. Unijunction transistor66 is a negative resistance device in that over a portion of itsoperating range when the emitter to base b1 current increases, thevoltage from emitter to base b1 decreases. In the circuit shown,capacitor 69 charges through resistor 67 from bus 38 until the negativeresistance region of unijunction transistor 66 is reached. Base b2 isconnected to a regulated voltage supply network comprised of resistor 70and voltage reference diode 71. At this point, unijunction transistor 66conducts and the voltage from emitter to base b1 drops sharply.Capacitor 69 i's then discharged through unijunction transistor 66. Thesaw-tooth waveform provided by the charge and discharge of capacitor 69is coupled to summing input 41 through voltage division networkcomprised of resistors 68a and 6811, and through capacitor 72, therebyremoving the D.C. level. The frequency of the oscillator output dependsupon the time constant of the RC combination, the voltage level of thebus, and is preferably relatively high; e.g., about one kilocycle.

The signals applied to summing input 41 from resistors 58 and 5'9consists of a voltage proportional to the output of generator on whichis superimposed the zeroreferenced saw-tooth waveform from theunijunction transistor `66 oscillator. The negative portions of thesawtooth component of this combined signal cause the voltage level atsumming input `41 to fall below the level of reference input 42,triggering operational amplifier 40 to its higher voltage output stablestate. This turns on transistors 52 and S3 and injects a pulse ofexcitation current into shunt field winding 13 of local generator S.Under normal operating conditions, the output voltage of local generator5 will be such that the generator output-proportional voltage at summinginput 41 will be slightly greater due to the superimposition of thesawtooth waveform than the voltage at the reference input 42.

The operation of the field excitation control element of the systemunder three different operating conditions is illustrated in FIG. 3 ofthe drawings. FIG. 3A shows voltage waveforms of the summing input 41and the reference input 42 and FIG. 3B shows the corresponding voltagewaveform at output 43 of operational amplifier 40. In FIG. 3A, line 73represents the voltage level at reference input 42. Line 74 representsthe output-proportional D.C. voltage at summing input 41 on which issuperimposed A.C. saw-tooth signal 75 from unijunction transistor 66oscillator. The output voltage of the local generator is substantiallygreater than the reference level. Whenever saw-tooth signal 75 dropsbelow the reference level 73, operational amplifier 40 causes Darlingtonpair 52 and 53 to conduct; and, when saw-tooth signal 75 rises abovereference level 73, operational amplifier 40` causes Darlington pair 52and S3 to turn off. FIG. 3B illustrates the waveform 76 at the output ofoperational amplifier 40 which corresponds to the inputs illustrated inFIG. 3A. Field winding 13 receives excitation current during thepositive portions of waveform 76,

FIGS. 3C and 3D illustrate waveforms corresponding to those in FIGS. 3Aand 3B but when the output voltage of the local generator is almostequal to the desired or reference level. The corresponding outputwaveform 0f FIG. 3D shows that shunt field winding 13 receives longercurrent pulses, thereby increasing the excitation to local generator 5.

As the voltage level represented by line 74 falls below the referencelevel depicted by line 73, the duty cycle of excitation pulses exceedsfifty percent and approaches the condition of constant excitation. Theinput and output voltage waveforms corresponding to this operationalcondition are shown in FIGS. 3E and 3F, respectively.

The duty cycle and level of excitation for a given generator may also becontrolled by changing the slope and amplitude of saw-tooth signal 75.

Through the use of pulse width modulated control of the excitationmeans, voltage regulation is accomplished in an improved and in a smoothand efiicient manner with a minimum of overshoot and oscillation orhunting of the output voltage of the local generator.

In previous on-off type voltage regulators, the relative direction ofthe on and off states of the field excitation means was determinedsolely by the output level of the generator relative to the desiredlevel. In such a case, the local generator would call for excitation orno excitation at its inherent speed of response to the excitation. Anon-off frequency of the excitation means was thus establishedcorresponding to the natural resonant response frequency of the localgenerator. This frequency was normally much lower than the preferredfrequency of the saw-tooth function signal described above in connectionwith this invention and, therefore, regulation of the output voltage wasnot as smooth as that possible with controls embodying this invention.This phenomenon is an even greater problem when generators are used inparallel. When two or more generators controlled in the prior artfashion described above are paralleled to the same load, their normallydifferent inherent speeds of response to excitation cause a loadunbalance which oscillates and beats between the generators. Eachparalleled generator effectively overshoots its varying share of theload in both directions.

In a parallel system of generators and controls embodying thisinvention, that problem is solved by having the generators excited bypulses of current preferably at a rate of approximately one thousandcycles per second. The rate of excitation is constant and the same forall generators and is much higher than the natural response frequency ofthe local generators. At this higher rate, the generators have asubstantially reduced time to overshoot and the electromagnetic andmechanical systems of the local generator cannot over-respond to it.Also, as previously described, diode 57 tends to maintain a smooth fiowof current through field winding 13 by providing a path for the currentinduced in field winding 13 during the off time of the excitation means.

The third input through resistor 60 to summing input 41 of operationalamplifier 40 is for the purpose of load equalization between localgenerator 5 and the other generators on the line. This input is providedby an operational amplifier 77 which detects differences, if any, in theamount of load being supplied by local generator and the othergenerators on the line. Operational amplifier 77 is connected as amodified differential amplifier having feedback paths through resistor78 and its parallelconnected bypass capacitor to its inverting input 79,and through resistor 92 and its parallel-connected bypass capacitor toits non-inverting input 81 from reference voltage source ER. Operationalamplifier 77 also has an inverting input resistor 80 and a non-invertinginput 81 and input resistor 82. Inputs 79 and 81 are tied through theirrespective input resistors to the opposite ends of a resistor 83inserted in series between generator terminal of local generator 5 andequalizer bus E. Connection to equalizer bus E is made through contact84 of equalizer relay 27.

The relative amounts of current being supplied by local generator 5 andthe other paralleled generators are determined by comparing the voltageappearing at terminal 15 with the voltage appearing at correspondingterminals of the other paralleled generators. As increasing current issupplied by local generator 5, for example, to load bus L, the voltageat generator terminal 15 becomes increasingly negative with respect toneutral terminal 17. By connecting the resistor 83 between generatorterminal 15 and the equalizer bus E, the magnitude and polarity ofV anydifference in voltage between terminal 15 and the equalizer bus issensed across resistor 83. Resistor 93 provides decoupling of Zenerdiode 85 from terminal 15 of the generator.

The voltage level at terminal 15 of the generator, however, is below thecommon mode input range of operational amplifier 77. In order to raisethe common voltage level at resistor 83 to the voltage level required bythe input of operational amplifier 77, Zener diodes 85 and 86 areconnected in series with resistors 87 and 88, respectively, and topositive bus 38. Zener diodes 85 and 86 have identical values of Zenervoltage so that the voltage across resistor 83 is transferred to theinput resistors 80 and 82 of operational amplifier 77 with itS relativeymagnitude and polarity preserved, but transformed in level to theoperating range of operational amplifier 77. The output voltage ofoperational amplifier 77 appearing at 89 and through filter networkresistor 94 and capacitor 95 comprises the third signal for the summinginput of operational amplifier 40 described above. This filtered outputis connected by line 91 to input resistor 60 at summing input 41. Outputat line 91 is also connected through equalizer relay contact 90 to A-Ain FIG. 5 described below in connection with the protection circuit.

In operation, if local generator 5 provides more than its share ofcurrent to load bus L, the voltage at generator terminal 15 will be morenegative than the voltage at the equalizer bus E. This voltagedifference will be developed across resistor 83 and transferred to theinput resistors 80 and 82 of operational amplier 77. The voltage atinverting input 79 will be negative with respect to the voltage atnon-inverting input 81, causing output voltage at 89 of operationalamplifier 77 to go positive. The preferable ratio of values of resistors78 and 80 is such that the gain of operational amplifier 77 is nominallyfifty. The positive signal appearing at output 89 is then transmittedthrough resistor input 60 to summing input 41 of operational amplifier40 where it is algebraically combined with the D.C. input that isproportional to local generator 5 output and with the saw-tooth functionsignal described above. The D.C. voltage level at summing input 41 thustends to become more positive with respect to the voltage at referenceinput 42, thereby decreasing the Width of each pulse of excitationcurrent transmitted to field winding 13 in the manner described above inconnection with FIGS. 3A and 3F. The excitation to field winding 13 isthus decreased and the load carried by local generator 5 is reduced tothe level of load carried by the other parallel generators.

If the voltage at terminal 15 of local generator 5 is positive withrespect to the voltage at equalizer bus E, the

voltage developed across resistor 83 makes inverting input 79 ofoperational amplifier 77 positive with respect to non-inverting input81. As a result, output 89 of operational amplifier 77 goes negative andcauses the voltage level at summing input 41 of operational amplier 40to go more negative with respect to reference input 42. AS describedabove, such a change in the relationship of the levels of the D.C. inputand the reference input increases the width of the excitation pulses.Field winding 13 receives longer pulses of current at the one thousandcycle per second rate, thereby increasing the level of field excitationto local generator 5. Local generator 5 will thus tend to contributemore current to load bus L.

From the foregoing, it is apparent that operational arnplifier 77 andits associated circuitry attempt to sum to zero the voltage differencebetween terminal 15 of local generator 5 and the equalizer bus E.

Another feature of this invention is embodied in the field excitationcontrol circuit described above and shown in FIG. 2. This featureincludes means to insure that local generator 5 under control of thefield excitation circuit is fully and continuously excited to the extentof its output in the event its output voltage is pulled down by a faultor other cause to and below the level required to operate the controlcircuitry. As long as the output voltage of the generator remains highenough to operate the control circuitry, the field excitation iscontrolled by the circuit in the manner described above. Such anoperating characteristic is normally preferred in aircraft D.C.generator power system as well as other systems.

Referring to FIG. 2, this failure condition operating characteristic isprovided by Zener diode 44 connected in series with resistors 45 and 47between the output of operational amplifier 4t) and the generator outputas it appears at positive bus 38. As described above, the conductingstate of Zener diode 44 controls the excitation of field 13 of thegenerator through connection of base 46 of transistor 48 to the junctionof resistors 45 and 47. When Zener diode 44 conducts, field 13 isde-energized; when Zener diode 44 blocks, field 13 is excited. Zenerdiode 44 conducts when the voltage applied to its cathode from theoutput of the generator through positive bus 38 and resistors 45 and 47exceeds the voltage applied to its anode by operational amplifier 40 `byan amount greater than the Zener voltage of the diode.

The Zener diode is selected so that, under normal operating conditionswith the generator output voltage maintained at or near the referencevoltage ER and no substantial faults present tending to pull down theoutput voltage, conduction of the diode is controlled by the stablestate of operational amplifier 40. When the output of operationalamplifier 40 is less positive, the difference between it and thesubstantially fixed generator output voltage exceeds the Zener voltageof diode 44 and it conducts. When operational amplifier 40 is in itsother stable state providing a more positive output, the appliedvolttage difference across Zener diode 44 is less than its Zener voltageand it blocks.

Under large fault conditions which drag the generator output voltagedown below the level required to operate operational amplifier 40 andthe control circuitry, operational amplifier 40 provides a less positiveoutput which normally calls for de-energization of the generator field,or the opposite of the desired function under the fault conditions. Asthe generator output voltage is brought down by the fault, however, thedifference between it and even the operational amplifiers less positiveoutput voltage falls below the Zener voltage of Zener diode 44 and field13 of the generator is now continuously excited to the extent possibleby the declining or reduced generator output. Thus, the advantage ofthis preferred operating characteristic for a large fault currentcondition is provided by this invention as embodied, for example, in thecircuit of FIG. 2.

11 AUTOMATIC LINE OONTACTOR CONTROL crRcUrr FIG. 4 illustrates inschematic form a preferred embodiment of the automatic line contactorcontrol circuit of this invention connected in operating relationship tolocal generator 5. Broadly, the `function of the automatic linecontactor control circuit is to control the flow of current to linecontactor coil 21 and equalizer relay coil 32, thereby controlling theconnection of local generator 5 to load bus L and to the equalizer busE. Local generator 5 must be connected to or disconnected from load IbusL and equalizer bus E at various times and for various reasons as willbe more -fully discussed below.

4In FIG. 4, the automatic line contactor control circuit comprises anoperational amplifier 96 connected as an integrator having a feedbackcapacitor 97, inverting input 98 and resistor 99, and non-invertinginput 100 and resistor 101. Inverting input 98 is shown monitoring theoutput voltage of generator 5 through voltage dividing resistors 102 and104 connected in series between the point of regulation, as shown, andneutral terminal 17. Non-inverting input 100 monitors the voltagebetween load bus L and generator terminal of local generator 5 throughvoltage-dividing resistors 105, 106, and 107.

During build up of local generator 5, main actuator 19l and equalizerrelay 27 are open, isolating local generator 5 from load bus L andequalizer bus E. Before main contactor 19 is allowed to close connectinglocal generator 5 to load bus L, the automatic line contactor controlcircuit must determine that the output voltage of local generator 5 iseither greater than, or only Very slightly less than, the voltage onload bus L so that local generator 5 can supply current to load bus Linstead of taking current from it. This determination is accomplishedand indicated by operational amplifier 96 as described below.

When main contactor 19 is open and local generator 5 is not supplyingthe load, the generator delivers only enough current to energize itsassociated control circuitry. This is a relatively small amount ofcurrent. As a consequence, the voltage at terminal 15 of local generator5 will then be very close to ground potential. In such case,non-inverting input 100 effectively monitors the voltage level aboveground of load bus L. Inverting input 98 monitors the output voltage oflocal generator 5. A voltage proportional to the difference betweenlocal generator 5 output and load bus L appears then between theinverting and non-inverting terminals 98 and 100, respectively, ofoperational amplifier 96.

When the voltage at inverting input 98 becomes positive or only veryslightly negative with respect to the voltage appearing at non-invertinginput 100, operational arnplifier 96 begins to integrate in the negativedirection and continues to integrate in the negative direction as longas this input voltage relationship exists. The slope of the outputsignal is determined by the time constant of feedlback capacitor 97 andinverting input resistor 99 and filter network resistor 111 andcapacitor 143 and the magnitude ofthe voltage difference at the inputs.

The output 108 of operational amplifier 96 is coupled to the summing andinverting input 109 of operational amplifier 110 through resistors 111and 103. Resistor 134 and diode 135 are returned to terminal 160 ofamplifier 161 in FIG. 5. Operational amplifier 110 is connected tooperate as a bistable device. Non-inverting input resistor 113 isconnected to a voltage reference 114 through resistor 115 and to output116 of operational amplifier 110 through resistor 117 in the feedbackconnection. Feedback resistor 117 functions to supply positive feedbackto non-inverting input 112, thereby enhancing the step function outputresponse of operational amplifier 110. Output 116 of operationalamplifier 110 is connected to the base of transistor 121 through theseries combination of resistor 118 and Zener diode 119. Transistor 121responds to the conductive state of Zener diode 119 in the same fashionas transistor 48 in the field excitation control circuit described abovein connection with FIG. 2. The base 120 of transistor 121 is alsoconnected to load bus L through a bleeder resistor 11711. The collectorof transistor 121 is tied to ground through the series combination ofresistor 123 and Zener diode 124. A bypass to ground around Zener diode124 is provided by NPN transistor 127. The collectors of transistors 125and 126 are connected through resistor 128 to the base of transistor 127for controlling its conductive state. Transistor 121 is connected to andcontrols the conductive state of a Darlington-connected pair of PNPtransistors 125 and 126.

Positive bus 38 is connected to or disconnected from actuating coil 21of main contactor 19 and through it to ground by the switching operationof PNP transistor 126 and contact 129 of field relay 26, and manualcontrol switch 181. When transistor 126 conducts and contact 129 andswitch 181 are closed, actuating coil 21 of main contactor 19 isenergized. When either or both transistor 126 and contact 129 are open,actuating coil 21 is de-energized. Opening of manual switch 181 willalso result in the de-energization of actuating coil 21 of the linecontactor. Actuating coil 32 of equalizer relay 27 is connected directlyin parallel with actuating coil 21 and with a bypass diode as shown.Actuating coil 131 of start fault relay 28 and its associated bypassdiode 183 are also connected in parallel through a blocking diode 132which functions with contact 133 of start fault relay 28 to lock inrelay 28 from positive bus 38 without energizing coils 32 and 21.

In operation, when the output voltage of operational amplifier 96 causesthe voltage level at the inverting input 109 of operational amplifier110 to become more negative than the voltage level at noninverting input112, the output of operational amplifier 110 will be a positivegoingstep function blocking Zener diode 119 which, in turn, turns offtransistor 121. When transistor 121 turns off, the voltage at thecollector of transistor 121 becomes less positive, thereby turning onDarlington pair 125 and 126 and allowing the base current of transistor125 to ow to ground through resistor 123 and Zener diode 124. WhenDarlington pair 125 and 126 is caused to conduct, the voltage levelappearing at its collectors causes transistor 127 to conduct, therebyshort circuiting Zener diode 124 and cutting it ofi'. Transistor 127also provides a path for the base current of transistor 125, therebytending to hold the Darlington pair conducting. When Darlington pair 12Sand 126 is conducting, current flows through contact 129 of field relay26 to energize coil 21 of line contactor 19, coil 32 of equalizer relay27 and coil 131 of start fault relay 28 which locks in through its owncontact 133. Main contactor 19 and equalizer relay 27 are thus caused toconnect local generator 5 to load bus L and to the equalizer bus E whenthe generator output voltage at inverting input 98 of operationalamplifier 96 equals or is only slightly less negative than the load busL voltage sensed by noninverting input 100.

Once main contactor 19 is closed and local generator 5 connected to loadbus L, operational amplifier 96 and its associated circuitry perform thefunction of detecting reverse current through local generator 5.Ideally, when local generator 5 is connected to load bus L, there is novoltage difference existing between load bus L and output terminal 14.Under normal circumstances, the field excitation control circuitequalizes any difference in the load shared between local generator 5and the other parallel generators on the line as explained above. Iflocal generator 5 does become a load on the load bus L, however, it isdesirable to take it off the line. This function is accomplished in thefollowing manner.

'Current supplied or taken by local generator 5 causes a correspondingvoltage drop across interpole Winding 12. The magnitude and direction ofcurrent supplied or taken by local generator 5 may thus be determined bymonitoring the magnitude and polarity of the Voltage level appearing atterminal 15 of local generator 5. As described above, when operationalamplifier 96 is operating to sense the voltage difference between loadbus L and the output of local generator for automatic line contractorcontrol, the voltage appearing at terminal is essentially groundpotential. In the reverse current sensing mode, the voltage of the loadbus L and of local generator 5 are normally equal; but, should there bereverse current flowing through the interpole winding 12, the voltageappearing at terminal 15 becomes positive with respect to ground,causing an increase in the voltage level at noninverting input 100 ofoperational amplifier 96 with respect to inverting input 9S. The outputof operational amplifier 96 will then begin to integrate in the positivedirection at a rate determined by the time constant of capacitor 97 andresistor 99', and filter resistor 111 and capacitor 143, and themagnitude of the differential input signal.

When the voltage at inverting input 109 from the output 108 ofoperational amplifier 9'6 becomes more positive than the referencevoltage at noninverting input 112 from voltage reference 114, the outputof operational amplifier 110 will be a negative-going step functionpermitting Zener diode 119 to conduct which, in turn, causes transistor121 to conduct through transistor 127. Darlington pair 125 and 126 isthen cut off which, in turn, cuts off transistor 127, causing Zenerdiode 124 to conduct current from transistor 121. When Darlington pair125 and 1216 stop conducting, actuating coil 21 of main line contactor19 and actuating coil 32 of equalizer relay 27 will no longer beenergized causing main contactor 19 and equalizer relay 27 to open.Contact 129 of field relay 26 remains closed so long as field relay 26is not tripped as will be discussed more fully below. In this manner,generator 5 is removed from the line and will remain off the line untilthe desired relationship between the generator output voltage and theload bus voltage is reestablished.

Start fault relay 28 is energized through its own contact 133 and willrelease only when the output voltage of local generator 5 falls belowthe hold-in current of coil 131. Diode 132 isolates coil 21 of maincontactor 19 and coil 32 of equalizer relay 27 from the output of localgenerator 5. The function of start fault relay 28 is to provide a pathfor detecting local ground faults before local generator 5 has beenconnected to load bus L so that such connection can be prevented untilthe local ground fault is cleared. This operation will be described morefully below.

The summing and inverting input 109 to operational amplifier 110 has anadditional input through resistor 134 and diode 135 from the startground fault sensing portion of the field relay trip control circuit. Inthe event of a local ground fault prior to closing of main contactor 19,this input operates to keep local generator 5 disconnected from load busL by preventing main contactor 119 and equalizer relay 27 from closingas will be more fully discussed below. It will be noted that a localground fault will cause current fiow through interpole winding 12 todrive operational amplifier 96 into its less positive state. Such anoutput, in turn, tends to cause second-stage operational amplifier 110to make Darlington pair 125 and 126 conduct and close the main contactor19. Since it is undesirable to put a local generator with a local groundfault in its local network to the line, the additional input signal toinverting input 109 is provided from the ground fault sensing section ofthe system.

yIt will be noted that Zener diode 119 functions to provide a fail-safetype of operating characteristic to the automatic line contactor controlcircuit of FIG. 4 and which is similar to the operating characteristicprovided by Zener diode -44 in the field excitation control circuitpreviously discussed. When local generator 5 is connected to load bus L,a remote ground fault can cause the load bus voltage to fall below thelevel required for normal operation of the control circuits. In such acase, Zener diode 119 will hold transistor 121 in the off state, therebycausing Darlington pair 125 and 126 to conduct and hold main contactor19 and equalizer relay 27 energized. Local generator S thus is lockedonto load bus L to assist the other paralleled generators in burning outthe remote ground fault.

Zener diode 124 also provides a fail-safe type operating characteristicbut does so in the opposite case from that in which Zener diode 119functions. If local generator 5 is tripped off the load bus L because ofsome fault, Zener diode 124 prevents Darlington-connected transistors125 and 126 from conducting and connecting local generator 5 to load busL until the output voltage of local generator 5 is at least equal to theZener voltage of Zener diode 124. From the foregoing description of thecircuit of the line contactor control, it will be apparent that, if theoutput voltage of local generator 5 is below the Zener voltage of Zenerdiode 124, transistors 125 and 126 are prevented from conducting forlack of a base current path from Darlington pair 125 and 126 unlesstransistor 127 is caused to conduct. However, Darlington pair 125 and126 must conduct in order to turn on transistor 127. Darlington pair 125and 126 is thus held in the non-conducting state, holding localgenerator 5 off load bus L until the local generator output voltage atleast equals the Zener voltage of Zener diode 124.

Thus, there as at least two operating conditions that must be satisfiedbefore local generator 5 can be connected to load bus L by maincontactor 19. First, the output voltage of local generator 5 must besubstantially equal to the voltage of load bus L; and, second, theoutput voltage of local generator 5 must be at least equal to the Zener`voltage of Zener diode 124. As a consequence, when a voltage-reducingfault occurs, Zener diodes 119 and 124 hold ymain contact 19 in itscondition at the time of the fault and until the fault is corrected.Lastly, local generator 5 cannot be connected to load bus L if a localground fault exists provided the ground fault signal is furnishedinverting input 109 of second-stage operational amplifier as shown inthe circuit drawing.

FIELD RELAY TRIP CONTROL CIRCUIT FIG. 5 illustrates in schematic form apreferred embodiment of the field relay trip control circuit of thisinvention in its operational relationship to local generator 5. Broadly,the function of this circuit is to trip field relay 26 in the event thata malfunction of some type occurs. Field relay 21 is a latching typerelay having a reset coil 31 and a trip coil 30. The field relay tripcontrol circuit controls the energization of field relay trip coil 30.When field relay 26 is tripped, shunt field winding 13 of localgenerator 5 is disconnected from its source of excitation, and maincontactor 19, equalizer relay 27 and start fault relay 28 a'rede-energized, causing local generator 5 to be completely disconnectedfrom the parallel generator system yIn FIG. 5, operational amplifier 136is connected as an integrator having feedback capacitor 137 and feedbackdiode 138. Connected to summing and inverting input 139 are summinginput resistors 140, 141, and 142; and connected to non-inverting input144- through resistor 145 is reference voltage 146. Feedback diode 138serves as a clamp, allowing operational amplifier 136 to integrate onlyin the negative direction. When the voltage level at the inverting input139 of operational amplifier 136 becomes more positive than thereference voltage level at nou-inverting input 144, the 'output ofoperational amplifier 136 begins to integrate in the negative direction.The rate of integration depends upon the time constant of feedbackcapacitor 137 and summing input resistors 140, 141, and 142 and upon themagnitude of the input voltage. If the summed voltage level at invertinginput 139 continues to be more positive than the reference voltage levelat non-inverting input 144, the output of operational amplifier 136continues to integrate in the negative direction until the anode voltageapplied to Zener diode 148 is lowered enough to reach its Zener voltageand render Zener diode 148 conducting. Feedback and Zener diodes 138 and148, respectively, effectively define the limits of integration ofoperational amplifier 136.

Output 147 of operational amplifier 136 is connected to the anode ofZener diode 148, and the cathode of diode 1-48 is connected to thecathode of a silicon controlled rectifier 149. The cathode is alsoconnected through a blocking diode 150 to a voltage reference source151. The gate of SOR 149 is tied to source 151 on the cathode side ofblocking diode 150. The anode of SCR 149 is connected to the positivebus 38 through resistors 152 and 153 and trip coil 30 of field relay 26as shown. A free-wheeling bypass diode 154 is provided for trip coil 30.

When Zener riode 148 conducts, the voltage level at the cathode of SOR149' falls below the gate voltage, thereby causing SCR 149 to conductthrough Zener diode 148. Current through 'SCR 149 energizes trip coil 30of field relay 26, opening its contacts 56 and 129 (see FIGS. 2 and 4)and thereby disconnecting field winding 13 from its source of excitationand disconnecting local generator 5 from load bus L and from equalizerbus E. Field relay 26 must be reset by an operator to bring localgenerator 5 bac-k into normal operation.

The signals to which operational amplifier 136 responds are appliedthrough resistors 140, 1-41, and 142 and algebraically combined atinverting input 139. lResistor 140 is adjustably connected to resistor157 of a series combination of resistors 156-158 comprising a voltagedivider connected between positive bus 38 and ground.

The signal applied throug resistor 140 is proportional to the outputvoltage of local generator 5. Potentiometer resistance 157 is set sothat, when the output voltage of local generator '5 exceeds apredetermined upper limit, the voltage level at inverting input 139becomes more positive with respect to the voltage at non-inverting input144, causing the output of operational amplifier 136 to integrate in thenegative direction and resulting eventually in local generator 5 beingtripped off load bus L. In this manner, operational amplifier 136 sensesand provides protection against overvoltages not corrected by the fieldexcitation control circuit.

Variable input resistor y142 is connected through blocking diode .159 tothe equalizer section of the field excitation control circuit shown inFIG. 2. Connection is made to output 89 of differential operationalamplifier 77 through contact 90 of equalizer relay 27. As describedabove, a positive signal from operational amplifier 77 indicates thatlocal generator 5 is providing more than its share of current to loadbus L. Under normal circumstances, load regulation is accomplished bythe field excitation control circuit. However, if, because of somemalfunction, operational amplifier 40 of the field excitation controlcircuit does not regulate the load in the normal manner as describedabove, the output voltage of operational amplifier 77 continues to rise,causing the voltage level at inverting input 139 of operationalamplifier 136 to become more positive and eventually disconnecting localgenerator 5 from the parallel system. Thus, in the event of uncorrectedoverexcitation of local generator 5, operational amplifier 136 acts asauxiliary means to protect the local generator.

The signal to inverting input 139 of operational amplifier 13.6 throughresistor 141 is, as shown in FIG. 5, from the output 160 of operationalamplifier 161 which is connected as an integrator having feedbackcapacitor 162 and input resistors 163 and .164 to inverting input 165and non-inverting reference input 166, respectively. Inverting input 165and input resistor 163 are connected to generator terminal of localgenerator 5 through 16 Zener diode 167 and contacts 168 of start faultrelay 28, and to a source of regulated voltage comprising resistor 169and Zener diode 171, as shown in FIG. 5. Non-inverting input .166 andresistor 164 are connected to ground through Zener diode 172 and topositive bus 38 through resistor 173, all as shown in FIG. 5.

The function of operational amplifier 161 is to sense any local groundfault before local generator 5 is connected to load bus L. When such alocal ground fault exists, local generator 5 tends to supply largeamounts of current to the ground fault. The large amount of currentsupplied by local generator 5 will cause the voltage level at generatorterminal 15 to go more negative with respect to ground. This voltagedifference between generator terminal 15 and ground is sensed andintegrated by operational amplifier 161. If this condition continues,the output of operational amplifier 161 continues to integrate in thepositive direction making inverting input 1 39 of operational amplifier136 more positive which will eventually trip field relay 26 andde-energize shunt field winding .13 of local generator 5.

In the inputs to operational amplifier 161, there are preferablyprovided Zener diodes 167 and 172 to raise the common level of voltageto the operating input level of operational amplifier 161 and to providea predetermined voltage difference between the inputs. Since operationalamplifier 161 will begin to integrate in the positive direction wheneverthe voltage level to inverting input 165 is more negative than thevoltage level to noninverting reference input 166, Zener diodes 167 and172 are selected to allow for a predetermined amount of current to besupplied by local generator 5, and, therefore, a predetermined voltagedifference between generator terminal ,15 and ground, before inputterminal 165 becomes negative with respect to input terminal 166. TheZener voltage of Zener diode 167 is thus chosen to be higher than theZener voltage of Zener diode 172 by an amount equal to the voltagedifference between generator terminal 15 and ground produced by apredetermined allowable amount of current flow through interpole winding12. When the voltage drop exceeds this amount, however, the voltagelevel at inverting input 165 of operational amplifier 161 becomesnegative with respect to the level of non-inverting input A166 andoperational amplifier 161 integrates in a positive direction asintended.

The output of operational amplifier 1.6.1 is also fed through conductor174 to the inverting input 109 of operational amplifier of the automaticline contactor control circuit. As previously disclosed, this signalwill lock out main contactor 19 and prevent the connection of localgenerator 5 to load bus L in the event of a local ground fault.

Contact 168 is a normally closed contact of start fault relay 28 which,as previously disclosed, is energized with the main contactor coil. Whenmain contactor 19 is actuated by the automatic line contactor controlcircuit, start fault relay 28 will also be actuated, causing contact 168to open so that operational amplifier 161 will no longer be adapted torespond to voltage difference between generator terminal k15 and ground.

Since operational amplifier .161 is no longer adapted to sense a localground fault when local generator 5 is connected to load bus L, thatfunction is now performed by current transformers 22 and 23 and theirassociated circuitry. In a typical use of the generator system hereindiscussed, each local generator 5 may be located physically aconsiderable distance from the point where it is connected by maincontactor 19 to load bus L so that a considerable length of heavy cableis required. The cable can develop a short circuit and/or becomegrounded at any point along its route due to any of a number of causes.To monitor this cable for such local faults, current transformers 22 and23 are inductively associated in the manner shown with the generatorcurrent path at widely spaced locations from each other and therebyprovide a protected zone between them. In FIG. of the drawings, thedesired interconnection of the transformers is shown by the dot symbolconvention applied to the secondary windings of current transformers 22and 23 together with the arrows indicating direction of current fiow inthe primary or load current conductor. Current transformer 23 is placedto Sense a change in the current in the return path to local generator 5while current transformer 22 is placed to sense a change in thegenerator current at a point in the cable as near as possible to linecontactor 19. Both current transformers 22 and 23 should sense the sameamount of change. However, if some local fault has developed in the zoneprotected by current transformers 22 aud 23, a current change occurs inone portion of the line which is not balanced by a corresponding currentchange in the other portion of the line. This unbalanced condition issensed by the current transformer and translated into :a differentialvoltage developed across resistor 175. Resistor 175 is connected acrossa rectifier bridge 176 which, in turn, provides a directed gate signalto SCR 177. The cathode of SCR 177 is connected to ground. Its anode isconnected through Zener diode 178 to trip coil 30 of field relay 26. Adifferential voltage of either polarity appearing across resistor 175 isconducted-by bridge 176 as a positive signal to the gate of SCR 177. Ifthe signal is of sufficient magnitude, SCR 177 is gated on, trippingfield relay 26 and disconnecting local generator 5 from the parallelsystem. lResistor .179 and capacitor 1-80 serve as a noise filter toprevent the turn on of SCR 177 by a spurious noise signal.

The foregoing description of a preferred embodiment of this invention isdivided into sections and the embodiment, therefore, is treated insections. This has been done this way as a convenience in presenting anddescribing the whole embodiment. The Various sections or subcombinationswork and operate together as an integrated whole. While thesubcombinations themselves are novel and enjoy their own features andadvatages as generator control and protection subcombinations for aparallel D.C. generator system, they are also novel when taken togetherand as a total system for controlling and protecting a parallel systemof D.C. generators and, in their total combination, they enjoy variousnovel features and advantages as is pointed out in the foregoingdescription.

Those skilled in the art will appreciate that various changes andmodifications can be made in the apparatus described herein Withoutdeparting from the spirit and scope of the invention.

I claim:

1. Apparatus for controlling and protectlng a D.C. generator connectedin parallel with other D.C. generators to a main load bus and to eachother through an equalizer bus said controlled and protected D.C.generator having an output terminal,

a grounded terminal,

an interpole Winding having one end connected to said armature at ajunction terminal and the other end to said grounded terminal,

a field winding terminal,

a field Winding having one end connected to said junction terminal andthe other end to said field winding terminal, and

an armature in circuit between said output terminal and said junctionterminal,

said controlled and protected D.C. generator having associated therewitha main conductor with actuating coil to connect and disconnect saidoutput terminal of said generator to and from the main bus,

an equalizer relay with actuating coil to connect and disconnect saidjunction terminal of said generator to and from the equalizer bus, and

a field relay having a reset coil to connect and a trip coil todisconnect said field winding terminal to and from said output terminalof said generator to energize and de-energize, respectively, saidgenerator field,

said apparatus comprising means for sensing the output voltage level ofsaid generator and providing an output voltage signal proportionalthereto,

means for sensing the voltage of said main bus and providing a main busvoltage signal proportional thereto,

means for sensing current flow to and from said generator in saidequalizer bus and providing an equalizer bus voltage signal proportionalthereto,

means for sensing direction of current flow through said generator andproviding a reverse current flow voltage signal proportional thereto,and

reference voltage means for providing a predetermined reference voltagesignal,

a field excitation control section comprising rst static means foralgebraically combining said generator output voltage signal and saidequalizer bus voltage signal and providing an output signal proportionalto the combination, and

first static comparing and bistable switching means having a first inputconnected to the output of said first static means, and a second inputconnected to said reference voltage means, and having an outputconnecting said field winding and said generator output for controllingthe energization of said field relay, said first comparing and -bistableswitching means being responsive to the relative levels of the outputsignal of said first static means and the reference voltage signal andassuming its first stable state energizing said field winding to excitesaid generator when the levels of the output signal of said first statemeans and said reference voltage differ in a first predetermined mannerand assuming its second stable state de-energizing said 4field windingto prevent excitation of said generator when the levels of the outputsignal of said first static means and said reference Voltage differ in asecond predetermined manner,

a main contactor control section comprising first static integratormeans having a differential input and responsive to input differences toprovide an integrated output signal from the input differences,

means for providing said generator output voltage signal and said mainbus voltage signal to said differential input of said first staticintegrator when said main contactor is open and disconnects said D.C.generator from said main bus, and for connecting the opposite ends ofsaid interpole winding to said differential input of said first staticintegrator to indicate voltage differential across said interpolewinding due to current flow therethrough when said main contactor isclosed and connects said D.C. generator to said main bus, and

second static comparing and bistable switching means having a firstinput connected to the output of said first static integrator means anda second input connected to said reference voltage means and having anoutput and additional switching means responsive to said output forenergizing and de-energizing said actuating coils of both said maincontactor and said equalizer relay when said second comparing andbistable switching means is in one and the other of its bistable statedrespectively, said second comparing and bistable switching means beingre- 3,546,532 is au sponsive to the output signals of said iirst statictrolling the connection and disconnection of said D.C. integrator meansgenerator to said main bus and said equalizer bus, whereby when saidmain contactor is open said said D C. generator having second bistablemeans assumes said one of its stable states closing said main contactorand said equalizer relay when said output signal of said first staticintegrator indicates the generator output voltage and main bus voltageare substantially the same and assumes the other of said stable statespreventing the closing of said said reference level by a predeterminedamount.

an armature,

an output terminal connected to one side of said armature,

a grounded terminal,

an interpole winding having one end connected to the other side of saidarmature at a junction terminal and the other end to said grounded maincontactor and said equalizer relay when lo terminal, said output signalof said first static integrator a field Winding terrninal, and indicatesthe generator output voltage and main a field Winding having one endconnected to said .bus Voltage differ by a predetermined amount,junction terminal and the other end to said field and, further, windingterminal, whereby when said main contactor is closed said said DC-generator having associated thereWith Second bistable means assumes Saidone of its a main contactor with actuating coil to connect and stablestates maintaining said main contactor disconnect 'said generator to andfrom the main and said equalizer relay closed when current buS, and nowthrough Said interpole winding is in the an equalizer relay withactuating coil to connect forward direction and assumes the other ofsaid and disconnect said junction terminal 0f Said stable states openingsaid main contactor and generator to and troni the equalizer bus, saidequalizer relay when current flow through Said circuit comprising saidinterpole winding is in the reverse direction, a reference voltagesource, a fault protection Section comprising a first operationamplifier connected as an intesecond static integrator means having adileren- Y grator and having an inverting and a non-inverttial input andresponsive to input differences to ing input, an output for providing anoutput provide an integrated output signal from the Signal, inputdifferences, rst voltage divider means connected between D.C. means forproviding to said differential input of generator output and ground Witha tap-Ofi t0 said second static integrator means a first input saidinverting input for providing a voltage in" signal proportional to thevoltage level of said Pnt Proportional to these output voltage of Saidgenerator output with respect to said generator D-C generator, groundedterminal, and a second input signal second Voltage divider meansconnected between proportional to the voltage level of Said generathemain bus and said junction terminal of said tor output with respect tosaid junction terminal D'C- generator and With a tap-off to Said nOnofsaid generator when said main contactor is inverting inPut for providinga voltage input open and disconnects said generator from saidproportional to theoutput voltage of said Inain main bus, bus, wherebysaid second static integrator means pro- 40 vvhereby When said linecontactor is open discon vides a first output signal proportional tocurrent necting said DC- generatOr from the main bus flow through saidinterpole winding and said the output of said iirst operationalamplifier generator when Said `generator is disconnected integrates in apositive direction when the level from Said main bus, of said main busvoltage input is greater than means providing said first output signalof said the level of said generator output voltage input, second staticintegrator means to said lirst input and integrates in a negativedireCtiOn When the of Said Second Statie bistable comparing and level ofsaid main bus voltage input is less than switching means to preventclosing said main the level of said generator output voltage input, linecontactor when current iiow through said and interpole winding exceeds apredetermined whereby when said line contactor is closed conamonnt,necting said D.C. generator to the main `bus second static means foralgebraically combining permitting said voltage dividers together tosaid generator output voltage signal, said out- Sense the Voltage dropacross said interpole put signal of said second static integrator, andWinding due to current flow therethrough, the said equalizer bus voltagesignal and providing output of said irst operational anipliiier inteanoutput signal proportional to the combinagrates in a positive directionwhen the level of tion, said inverting input voltage is less than thelevel third static comparing and bistable switching 0f Saidnon-inverting input voltage indicating a means having a iirst inputconnected to said reverse current flow through said D.C. generator,second static means and a second input conand integrates in a negativedirection when the nected to said reference voltage means and havlevelof said inverting input voltage is greater ing an output connected toenergize said trip than the level Of Said l10n-inverting input voltcoilof said field relay to disconnect said field age indicating a normalforward current ow winding from the output of said generator and throughsaid D.C. generator, said generator from said main bus and said G5 asecond operational arnpliiier having an inequalizer bus when said thirdbistable means is verting and a non-inverting input connected to in oneof its two stable states, said third bistable said output of said firstoperational amplifier means being responsive to input signals wherebyand to said reference voltage Source, respee. said trip coil isenergized when said combinatively, and an output for providing an outputtion output signal of said static means exceeds Signal, Said secondoperational amplifier being connected as a bistable switch providingpositive 2. In combination with a D.C. generator connected in parallelwith other D.C. generators to a main bus and to each other through anequalizer bus, an automatic line contactor and reverse current fiowsensing circuit for con- 7 and negative square wave output signals whensaid inverting input level is less and more, respectively, than saidnon-inverting input level, a static power switch directly controllingcurrent flow from said output of said D.C. generator output and saidactuating coils of said main contactor and said equalizer relay,

a static control switch connected to the output of said secondoperational amplifier and connected in controlling relationship to saidstatic power switch, said static control switch being responsive to saidoutput of said second operational amplifier,

whereby said control switch conducts when the output of said secondoperational amplifier is negative and blocks when the output of saidoperational amplitier is positive, and said power switch beingresponsive to the state of said control switch whereby said power switchconducts energizing current to said actuating coils of said linecontactor and said equalizer relay when said control switch blocks, andblocks energizing'current to said actuating coils of said line contactorand said equalizer relay when said control switch conducts.

3. The circuit of claim 2 in which said second operational amplifierpower requirements are provided from the output of said D.C. generatortogether with fail-safe means for insuring the continuous energizationof the actuating coils of said line contactor anfd said equalizer relayto maintain said D.C. generator on the line in the event of a remoteground fault which drops the system |voltage below said secondoperational amplifier power requirements, said fail-safe means includinga Zener diode having its anode connected to the output of said secondoperational amplifier and its cathode to said control switch and theoutput of said D.C. generator whereby said Zener diode blocks and saidD.C. generator output voltage turns off said control switch therebyturning on said power switch and energizing said actuating coils of saidline contactor and said equalizer relay when said second operationalamplifier output is positive, and whereby said Zener diode conductspreventing said D.C. generator voltage from turning` off said controlswitch thereby turning off said power switch and preventing theenergization of said actuating coils of said line contactor and saidequalizer relay when said second operational amplifier output isnegative, and

whereby said Zener diode provides a fixed amount corresponding to itsZener voltage by which said D.C. generator output voltage must exceedsaid second operational amplifier output voltage to permit said controlswitch to conduct thereby de-energizing said actuating coils of saidline contactor and said equalizer relay such that as said D.C. generatoroutput voltage falls below the power requirements of said secondoperationaI amplifier causing its output to become negative, said D.C.generator output voltage exceeds said negative output of said secondoperational amplifier by less than the Zener voltage of said Zenerdiode.

4. The circuit of claim 2 together with means for insuring said linecontactor is not closed unless said D.C. generator output exceeds apredetermined level, said means comprising a Zener diode having a Zenervoltage in circuit with said control switch and said power switch andsaid output of said generator and arranged to prevent said power switchfrom conducting when the output voltage of said generator is less thanthe Zener voltage of said Zener diode.

5. In combination with a D.C. generator connected in parallel with otherD.C. generators to a main 'bus and to each other through an equalizerbus, a field relay trip control circuit for preventing excitation ofsaid D.C. generator in the event of preselected fault and operating con-22 ditions,

said D.C. generator having an armature, an output terminal connected toone side of said armature, a grounded terminal, an interpole windinghaving one end connected to the other side of said armature at ajunction terminal and the other end to said grounded terminal, a fieldwinding terminal, and a field winding having one end connected to saidjunction terminal and the other end to said field winding terminal, saidD.C. generator having associated therewith a main contactor withactuating coil to connect and disconnect said generator to and from themain bus, and an equalizer relay with actuating coil to connect anddisconnect said junction terminal of said generatorto and from theequalizer bus, said circuit comprising a reference voltage source,static integrator means having a differential input and responsive toinput differences to provide an integrated output signal from the inputdifferences, means for providing to said differential input of saidstatic integrator means a first input signal proportional to the voltagelevel of said generator output with respect to said generator groundedterminal, and a second input signal proportional to the voltage level ofsaid generator output with respect to said junction terminal of saidgenerator when said main contactor is open and disconnects saidgenerator from said main bus, whereby said static integrator meansprovides a first output signal proportional to current flow through saidinterpole winding and said generator when said generator is disconnectedfrom said main bus, static means for algebraically combining saidgenerator output voltage signal and said output signal of said secondstatic integrator, and providing an output signal proportional to thecombination, static comparing and bistable switching means having afirst input connected to said static means and a second input connectedto said reference voltage source and having an output connected toenergize said trip coil of said field relay to disconnect said fieldwinding from the output of said generator and said generator from saidmain bus and said equalizer bus when said bistable switching means is inone of its two stable states, said bistable switching means beingresponsive to input signals whereby said trip coil is energized whensaid combination output signal of said static means exceeds saidreference level by a predetermined amount.

References Cited UNITED STATES PATENTS 3,225,284 12/1965YoshinoriKawaietal.-- 322-28 J D MILLER, Primary Examiner H. FENDELMAN,Assistant Examiner U.S. Cl. X.R. 317-18, 22, 26; 3212-28

